There are a couple of ways to do this.
You could use larger yarn, but you want to match the blanket.
You could use a larger needle and a slightly looser gauge. The limit on that will be whether the work still looks good at a larger size. In this case, you might be able to go up one needle size and get what you need. Here, look at the size charts on this site:http://www.bevscountrycottage.com/baby-sweaters.html

If you don't like the look of the bigger gauge, you can do the math and bump up the size. If you can find a sweater done in similar yarn and see how many stitches a 12-month size uses, you can try that on your pattern.